Glioblastoma (GBM) is the most common primary brain tumor and also the highest grade (WHO gradeIV). Progression to GBM represents an abrupt turning point, with death quickly following the transition. One of the most specific pathologic features that emerges during the transition and distinguishes GBM from lower grade tumors is necrosis with surrounding cellular "pseudopalisades". Pseudopalisades are composed of hypoxic tumor cells that secrete pro-angiogenic factors critical for promoting angiogenesis and tumor expansion. Mechanisms underlying the development of pseudopalisades, hypoxia, and necrosis in GBM are undefined, but we believe that understanding their origins will be critical for attempts to stabilize this disease. We hypothesize that vaso-occlusion and intravascular thrombosis give rise to pseudopalisades and the ensuing hypoxia-induced angiogenic cascade, accounting for the abrupt onset of rapid disease progression. Our preliminary data has demonstrated that thrombotic vascular occlusion within the neoplasm is associated with hypoxia-induced outward migration of glioma cells to form pseudopalisades. Mechanisms by which neoplastic cells induce endothelial damage, vaso-occlusion, and thrombosis have not been established. Ang- 2 is a Tie-2 receptor antagonist that mediates endothelial apoptosis in experimental gliomas and is a prime candidate for initiating these events. Since PTEN mutations occur during the transition to GBM, we will examine whether PTEN loss leads to the secretion of proteins that trigger endothelial apoptosis through Ang- 2. We also hypothesize that increased expression of the pro-thrombotic proteins tissue factor (TF) and protease activated receptor-1 (PAR1) promote intravascular thrombosis. We will examine whether PTEN loss or hypoxia promotes TF-mediated intravascular clotting and whether PAR1 activation leads to increased cellular migration associated with pseudopalisade formation. An animal model of astrocytoma will be used to validate the significance of PTEN loss, TF expression and intravascular thrombosis in the progression to GBM and to determine if anti-thrombotic therapies are capable of delaying the development of hypoxia and prolonging survival. Vaso-occlusion and intravascular thrombosis have not been previously recognized as driving forces in the development of hypoxia, angiogenesis and glioma progression. This proposal introduces entirely novel concepts that may explain the highly aggressive properties of GBM and suggests therapeutic approaches that could potentially stabilize its progression.